Printed circuit board assemblies are used in computers, communications equipment, televisions, and many other products. In a typical printed circuit board assembly, many electrical components are attached to the top and bottom surfaces of a printed circuit board ("PCB"). Since the electronics manufacturing industry is highly competitive, it is important to maximize the through-put of processing PCB assemblies and to securely attach the electrical components to the PCBs.
The manufacturing of PCB assemblies involves many processes, one of which is surface mounting components to PCBs. To surface mount components to a first surface of a PCB, a dispenser deposits a solder paste and/or an adhesive compound onto the first surface of the PCB, and then a pick-and-place machine presses the components against the solder/adhesive. After the first side of the PCB has been populated with components, the PCB is inverted and the process is repeated to surface mount components to the second side of the PCB. The solder/adhesive dispenser is typically a highly accurate stenciling machine, and the pick-and-place machine is typically a turret type device.
The manufacturing issues related to supporting the populated sides of PCB assemblies are best understood in light of the structure and operation of conventional stencil printing machines. Conventional stenciling machines generally have a stencil plate, a movable platform under the stencil plate, and a PCB holder attached to the platform. Conventional stenciling-machines also have two spaced-apart, parallel conveyor tracks to transport the PCB along a conveyor line between the stencil plate and the movable platform. In operation, the conveyor tracks engage opposing edges of the. PCB and transport the PCB until it is over the PCB holder, the work platform then moves upwardly to lift the PCB to the stencil plate, and a wiper moves across the stencil plate to press the solder paste or the adhesive compound onto the PCB. After the wiper stops, the work platform moves downwardly from the stencil plate to replace the PCB on the conveyor rails so that it can be transported to a pick-and-place machine.
In addition to maximizing the throughput of processing PCB assemblies, it is also becoming important to accurately fount a large number (e.g., 1000-1500) of very small components to one side of the PCB assemblies. Accordingly, an important aspect of stencil printing on PCBs is to accurately align a PCB with the stencil plate to deposit a pattern of solder/adhesive pads at desired locations on the upwardly facing surface of the PCB. Another important objective of stencil printing PCBs is to hold the PCB flat with respect to the stencil plate so that a desired volume of the solder/adhesive compound is deposited evenly across the upwardly facing surface of the PCB. Thus, it is generally desirable to securely attach the PCB to the PCB holder.
To securely attach the PCB to the PCB holder, conventional PCB holders typically have sidewalls projecting up from the platform to support the perimeter of the PCB, and spacers positioned within the sidewalls to support interior portions of the PCB. A vacuum is drawn within a chamber defined by the platform, the sidewalls and the PCB to securely attach the PCB to the sidewalls and draw the interior portion of the PCB against the spacers. It will be appreciated that the spacers prevent the PCB from bowing into the chamber by holding the PCB flat.
Conventional stenciling machines, however, encounter several manufacturing issues when components are surface mounted to both surfaces of PCB assemblies. Conventional stenciling machines are particularly difficult to operate efficiently in the manufacturing of contract PCB assemblies in which five or more runs of different PCB assemblies with different configurations of components are processed in a short period of time. For example, after the first side of a PCB is populated with components and inverted to stencil print the second side of the PCB, one problem with conventional PCB holders is that the spacers may engage some of the components on the first side of the PCB. To avoid this problem for processing several runs of PCBs with different configurations of components, the spacers are manually attached directly to vacant areas between components on the populated side of each PCB. It will be appreciated that manually attaching the spacers to each PCB is not only time-consuming, but an operator may also fail to attach the spacers to all of the desired support points on the PCB to provide adequate support during processing. Therefore, there is a significant need to quickly and consistently support PCB assemblies with different configurations of components in the manufacturing of contract PCB assemblies.